1. Field of the Invention
This invention relates to electrodeposited alloys and particularly to an electrodeposited eutectic tin-bismuth alloy on a conductive substrate.
2. Description of the Prior Art
As printed circuit boards (PCBs) become thicker and more complex, the need to minimize thermal stresses during processing, and the need to obtain completely filled solder holes, become conflicting requirements. In general, more difficult solder requirements are satisfied by increasing the soldering temperature of the board. Unfortunately, however, such an increase also increases the occurrence of board delamination and cracking of the plated "through holes," passages through the PCB for the purpose of emplacing wires or electronic components for carrying current from one face of the board to the other. In addition, rework of the solder on a PCB requires process temperatures and cycle times that increase with board thickness and complexity. Dissolution of the copper plating by molten solder during rework is an added detrimental factor in board survival and reliability.
Such high-melting eutectic tin-lead alloys, typically with compositions around 63 weight percent (wt. %) of tin and 37 wt. % lead, with a melting point around 183 degrees Centigrade (.degree.C.) have conventionally been used both as plated coatings for multilayer circuit-board assemblies and as a eutectic solder for bonding together the layers of tin-lead plated circuit boards. However, tin-lead alloys have not proven entirely satisfactory in some temperature-sensitive applications where heat is undesirable. The temperature required to melt eutectic tin-lead alloy is high enough to damage the components of the PCB, or to impair the conductivity characteristics of the board.
U.S. Pat. Nos. 4,717,460 and 4,565,610, both to Nobel, disclose the use of bismuth in small amounts in plating baths containing tin, lead and sulfonic acids. As little as 0.1 gram per liter (g/l) of bismuth is claimed to improve the low-current-density brightness of the plated tin-lead material if an aromatic aldehyde or alkylene oxide is also present in the bath. However, Nobel does not teach the utility of a tin-bismuth alloy per se.
Bismuth has also been used in combination with tin in electroplating systems. In some electroplating processes, small quantities of bismuth have been added to retard the formation of tin pest and tin whiskers. For these purposes, between about 1 and 2 wt. % of bismuth in the co-plated material, or co-plate, is ordinarily suitable. U.S. Pat. No. 4,331,518 to Wilson discloses an electroplating process which produces tin-bismuth alloys which may have at most not more than about 0.8 wt. % bismuth in the co-plate. Soluble bismuth is provided in the electroplating bath of that patent as a chelated acid bismuth sulfate gluconate.
It has now been discovered that the difficulties of the prior art associated with the use of tin-lead alloys can be overcome substantially, and in some cases completely, by replacing the lead by bismuth. Environmental considerations are also significantly improved by the replacement of lead with bismuth.